Phosphorescent Tris-Bidentate IrIII Complexes with N-Heterocyclic Carbene Scaffolds: Structural Diversity and Optical Properties

N-heterocyclic carbenes (NHCs) are highly versatile ligands with unique features that are able to engage into robust metal–ligand bonds, owing to their strong electron σ-donating and relatively weak π-accepting abilities. Aiming at efficient and stable emitters, coordination features of NHCs have been successfully employed for modulating both optical and redox properties yielding outstanding IrIII complexes. In addition, NHCs are an unparalleled tool that helps to suppressing nonradiative pathways and shift emission into the blue region. As a result, IrIII complexes with NHC ligands represent an appealing class of phosphorescent emitters that show great potential application in light-emitting devices, electro-chemiluminescence and biomedicine. Herein, the interplay between structural diversity and photophysical properties will be discussed for a colorful palette of IrIII-NHC complexes in a systematic approach. Finally, their employment in optoelectronic devices and biomedicine will be described.